A resonator fiber optic gyroscope (RFOG) is a rotation rate measurement apparatus that uses a fiber ring resonant cavity to enhance a rotation-induced Sagnac effect. The basic principle of RFOG operation is that the effective resonator round-trip path length in a clockwise (CW) and counter-clockwise (CCW) direction is different when the rotation has a nonzero component in a resonator axis. By measuring the CW and CCW resonance frequency difference, which is proportional to Sagnac phase shift due to rotation, the RFOG can accurately measure the rotation rate. Several RFOG configurations are suggested by the prior art. Three specific prior art resonator configurations are shown in FIGS. 1-3.
FIG. 1 shows a resonator in reflection mode configuration, where light is introduced to a resonator formed by a resonator input mirror and a fiber optic resonator coil. The mirror has some reflectivity and a low, but non-zero, transmission coefficient. Thus, most of the light incident on the mirror is reflected, but a portion of the light is transmitted. In operation, light from a laser is introduced to the resonator after being transmitted through the resonator input mirror. Light recirculates within the resonator multiple times by means of the resonator input mirror, i.e. light emerging from one end of the fiber is repeatedly reflected back into the other end. Some of the recirculated light is transmitted out of the resonator (dotted line) to the detector, where the recirculated light is interfered with light that was originally reflected from the light source (solid line toward the detector). These interfering lightwaves from the resonator input mirror are used to measure the resonance signal. Specifically referring to the light propagating in the CW direction of the coil in FIG. 1, a small portion of light from the CW laser enters the resonator at the resonator input mirror. Most of the light from the CW laser is reflected by the resonator input mirror and continues towards the CCW laser. A portion of the light that recirculates within the resonator coil is coupled out of the resonator along the same path as the reflected light. A portion of the reflected light and the resonator output light are redirected by the CW tap mirror to a CW detector. The reflected light and the resonator output light interfere on the CW detector. The interference results in a lightwave having a resonance dip corresponding to zero light. The bottom of the dip occurs when the frequency of the light is at a resonance frequency of the coil. Similarly, light from the CCW laser is introduced into the resonator in the opposite direction and its resonance is detected by the CCW detector.
A problem associated with the architecture shown in FIG. 1 is that large rotation sensing errors are caused by the interference between an undesirable portion of the reflected light and the resonator output light. The undesirable portion of the reflected wave could be slightly different in its spatial-mode or polarization-mode characteristics than that of the resonator output wave. Even after careful alignment, polarization dependent losses or spatial aperturing effects between the resonator input mirror and the detector can cause errors in the rotation rate measurement. For example, imperfections in the input polarization state will result in line-shape asymmetry, which in turn will result in a gyro rate bias error. Accordingly, there is a need to detect the resonance frequency of the resonator without interfering the reflected and resonator output waves.
FIG. 2 shows prior art of a resonator in transmission mode. This resonator architecture overcomes the problem with the previous reflection-mode resonator by placing another mirror within the cavity to tap off a portion of the light recirculating in the coil. Specifically, the light that is reflected by the resonator is removed by an optical isolator placed in front of the opposing laser. Thus, only the light recirculating in the resonator in the CW direction reaches the CW detector, i.e. no interference occurs between the recirculating light and the reflected light. However, this architecture is not symmetric in the CW and CCW directions because only some light in the CW direction propagates through the fiber coil prior to reaching the CW detector, while all of light in the CCW direction propagates through the fiber coil prior to reaching the CW detector. This asymmetry poses a problem. If some light propagating in the CW direction is not in the correct spatial-mode or polarization mode, it can leak through to the CW detector and may be different from the light that is recirculating within the resonator. Thus, detected light could still be mismatched to the resonator output light and reach the detector without first passing through the resonator coil. It is known that this asymmetry combined with polarization and spatial mode imperfections can lead to significant rotation sensing errors. Accordingly, there is a need for a symmetric, transmission mode resonator.
FIG. 3 shows a prior art illustration of a symmetric, transmission mode resonator. This resonator is formed by adding a third mirror to the resonator cavity. This resonator is both in transmission-mode configuration and symmetrical for the CW and CCW light propagation. In this configuration, the light reaching the detector always passes through the resonator coil fiber at least once, thus eliminating the issues discussed for the configurations shown in FIGS. 1 and 2. However, adding a third mirror adds significant complexity to the optics within the resonator cavity. To achieve high performance, the round trip optical loss within the cavity must be very low. Achieving low optical loss in the cavity is far more important than achieving low loss outside the cavity. Specifically, a round trip loss of under 1 dB is usually acceptable within the cavity, and up to three dB of loss is acceptable outside the cavity. The angular alignments of each cavity mirror are critical to achieving low loss in the cavity. Including the third mirror within the cavity increases the difficulty in obtaining a low cavity loss with a low cost device that is capable of being manufactured with a high degree of automation. Additionally, environmental changes may exacerbate the cavity loss because temperature changes can disrupt the alignments of the cavity mirror. Accordingly, there is a need for a simplified, symmetric, transmission mode resonator with only two mirrors.